1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus that has a rendering system using multi-light source simultaneous scanning, for example.
2. Description of the Related Art
With electrophotographic image forming apparatuses that use a plurality of light sources, it is possible to increase the scanning density when using an element having a wide light source interval by employing an oblique arrangement as shown in FIG. 1, for example. Since the positions of the light sources in the scanning direction are different in order to compensate for such an oblique arrangement, the timing of the driving signal for each of the light sources is also different. The beams of light from the respective light sources simultaneously scanned are concentrated on a small area spatially and temporally, which makes it difficult to separate synchronization signals using a sensor. Accordingly, it is difficult to achieve light source synchronization, which is for aligning a rendering position on a photosensitive member, for each of the individual light sources.
As a low-cost substitute method, there is a method in which only one light source is synchronized among multiple light sources, and a substitute is used for the remaining light sources. For example, assuming that the difference in scanning timing between light sources of the multiple light sources is always constant depending on the spatial arrangement of the light sources, by measuring the difference in scanning timing between the light sources in advance, the scanning timings of the remaining light sources can be calculated from the scanning timing of one light source used as a reference. The amount of timing delay is measured in advance using a sensor, for instance, and synchronization signals for scanning timings of the remaining light sources are generated from the synchronization signal of the one light source and the measured amount of delay (see Japanese Patent Laid-Open No. 2006-187868).
Further, in order to correct shift that has occurred from a difference between scanning angles, for instance, caused by the physical structure of an optical system or the like, a scanning timing of the actual optical system, and the scanning speed of scanning performed on the photosensitive member corresponding to the scanning timing are measured for each light source. Then, it is necessary to correct a rendering start time of one pixel for each light source, and the time difference in scanning timing between the light sources. As a measurement method, a method for performing two-point light measurement has been proposed (see Japanese Patent Laid-Open No. 2006-208697, for instance).
However, by increasing the number of light sources, with the method disclosed in Japanese Patent Laid-Open No. 2006-187868 as described above and the like, the scanning timings between the light sources do not fully match in many cases. Particularly, if a plurality of light sources are shifted and arranged as shown in FIG. 1, the optical path length (optical distance) to a photosensitive member varies for each light source. Accordingly, unlike single-light-source scanning, with multi-light-source scanning, due to fluctuation in the optical distance, fluctuation occurs in the difference in scanning timing between the light sources. Although this phenomenon is not very prominent with approximately two light sources, a problem with pixel shift in the main-scanning and sub-scanning directions occurs as the number of light sources increases. Further, with the method disclosed in Japanese Patent Laid-Open No. 2006-208697 and the like, a method according to which a plurality of scanning timing measurement means are provided results in a disadvantageous mechanical configuration in terms of cost and volume.